The present invention concerns a method for displaying a cycle of successive images of the moon during a lunation in a circular aperture of a dial by means of a mobile indicator arranged behind the aperture and having, on the face thereof that appears in the aperture, lines of separation of different shapes between light fields and dark fields for showing respectively the illuminated part and the dark part of the moon seen from the earth.
The invention also concerns a device for displaying a cycle of successive images of the moon during a lunation in a circular aperture of a dial, particularly for implementing the aforementioned method, comprising a mobile indicator arranged behind said aperture and driving means for moving the indicator step by step so that different parts of one strip-shaped region of the indicator can be seen in succession in the aperture. The invention further concerns a watch comprising a watch movement and this type of display device, whose drive means are controlled by the watch movement.
In conventional devices displaying the phases of the moon, a disc bearing two images of the full moon makes a half revolution per lunation behind a semi-circular aperture of a particular shape, illustrated for example in U.S. Pat. No. 508,467. One of the edges of the aperture comprises two convex arcs which go over the image of the full moon, respectively while the moon is waxing and waning. The shape of the image thus displayed is correct only at the start and at the end of the lunation (starting from the new moon), when the illuminated part has the shape of a crescent, and at full moon. During the other phases, the image displayed has an incorrect shape, since the shape of the line of separation between the light zone and the dark zone is not true to reality: it is curved instead of being straight at the first and last quarter, and it is curved in the wrong direction between the first and the last quarter. In other words, this display mode gives an image of the moon that is not true to reality for most of the lunation.
CH Patent No. 598 638 provides a method and a display device of the type indicated in the preamble hereinbefore, using a circular aperture in front of a disc which rotates through one step every two days about the axis of the hands of a watch and which, in this case, carries a series of fifteen successive images of the lunar disc as it is seen in the sky during the lunation. In practice, the space required for each image and the need to prevent the edge of a second image being seen in the aperture mean that the diameter of the aperture and the image has to be less than approximately one seventh of the diameter of the disc, thus particularly small on a wristwatch dial. These images conform better to reality than those of conventional devices, but their evolution remains quite inaccurate, since the display only changes approximately every two days. If one desired more frequent updating, the number of images on the disc would have to be doubled and their size thus reduced so much that the display would lose all appeal.